Sima de los Huesos (Bones Pit) belongs to the Cueva Mayor – Cueva del Silo karst complex. To enter the site now, scientists have to crawl for 500 metres through Cueva Mayor to a 13 metre deep pit. At the bottom lies one of the world’s most important fossil deposits, with a chronology spanning roughly 500,000 years. Never have so many human fossils from the same species been found in the same place.

Now we know the value of the site, but the early days were hard and full of uncertainty. In 1976, Trinidad Torres was collecting bear remains in the Sima when he picked up several human bone fragments as well. He gave them to Emiliano Aguirre, who immediately realised the potential of the cave and began a dig. In 1983, Emiliano’s team started to clear the Sima, which had been used as a rubbish tip, mixed with sediment. The intact section was only reached in 1984. Dozens of human fossils slowly appeared, and we began to appreciate the importance of the Bones Pit, not only because the material was qualitatively important, but also because of its quantitative scale. This site contains a large number of H. heidelbergensis individuals. Remains continued to be exhumed until 1992, when the big surprises began. The summer dig yielded a complete skull, known as Agamenón (Skull 4), Miguelón (Skull 5), the most complete Homo skull in the world’s fossil record, and Skull 6, which had to be restored in the lab because it came out in fragments. Bones from the middle ear also appeared, including anvils, hammers and stirrups. Miguelón’s jawbone was discovered the following summer.